Single layer and multilayer Ceramic transformers offer a number of advantages over standard electromagnetic based transformers including ease of miniaturization and high galvanic isolation. However, impediments exist that have made single layer and multilayer ceramic transformers unattractive in applications that generally employ standard isolated magnetic DC/AC voltage level shift devices.
Ceramic transformers can be categorized into three types: Rosen type, thickness vibration type, and planar vibration-type, displayed in prior art FIG. 1a, FIG. 1b, and FIG. 2a. Among the three types, Rosen type transformers are most common as they traditionally provide the highest voltage gain and power density for DC-DC or AC-DC converter applications. However, several major challenges have prevented wider acceptance of ceramic transformers.
Although ceramic transformers are inherently very highly power dense devices, power handling and power throughput are distinct issues. Existing ceramic transformers are of high power density, but the voltage range in which they operate optimally is too high for typical modern electronics. The issue is not one of power density (ceramic transformers are typically capable of 40-50 W/cm3). The issue is that the power throughput supply voltage requirements necessary to achieve appropriate power throughput are higher than normally supplied by conventional applications and that such high supply voltage is likely to rapidly cause internal damage to the ceramic transformer.
It is believed that one of the underlying reasons for this lack of power throughput performance is a lack of ability to design or control by construction both the supply side impedance and the load side impedance of the existing ceramic transformers. To obtain improved power throughput requires both the supply side impedance and the load side impedance must be simultaneously addressed. Prior art ceramic transformer designs, including multilayer ceramic transformer designs, do not exhibit the ability to adjust both the input and the output impedance characteristics to control the power throughput. Accordingly, there is a need for a ceramic transformer design that can simultaneously enable impedance selectivity at both input and output through construction selection as to both provide a more agile application capability, a superior power throughput capability and a lower cost of manufacture. The invention provides for such capability.
Another challenge to wide implementation of existing ceramic transformers is that it is difficult to design ceramic transformers for efficient step-down voltage gain at higher power. For example, there are fundamental problems at present in attempting to employ known ceramic transformer devices for low impedance (high current/low voltage) applications for small electronics devices such as cell phones and iPod chargers and similar such converter or isolator applications. In many existing ceramic transformers, the voltage gain automatically increases as the output load decreases. This leads to a difficulty in that increasing power capability for such ceramic transformers directly leads to impedance mismatch at low impedance output electrical loading conditions. What is needed is a method to provide moderate to large step down ratios in a manner that maximizes mechanical coupling efficiencies and enables significant power throughput.
Still another challenge of the prior art is that known ceramic transformers tend to be “gain specific”. Prior art ceramic transformers are generally either step-up type or, much more rarely, a step-down type of a small step-down ratio. A common approach to achieving step-down capability is simply to reverse the input and output connections. However, this approach leads to further limiting power throughput and a reduction in efficiency. What is needed is a method to provide both moderate and large power levels and step-down ratios in a single modular design that maximizes electrical power conversion efficiency and enables significant power throughput.
More recent developments in piezoelectric transformer technology, as exemplified in (a) Vo Viet et al, “Investigation of the Optimum Design for a 10 W Step-down 3-layer Piezoelectric Transformer,” Journal of the Korean Physical Society, 58, No. 3, March 2011 and (b) Kim, Insung et al, “Ring-dot-shaped Multilayer Piezoelectric Step-down Transformers Using PZT-based Ceramics”, Journal of the Korean Physical Society, Vol. 57, No. 4 (2010), have sought to use the advantage of unipoled transformer construction in multilayer designs, as disclosed in U.S. Pat. No. 5,278,471, as to enable step-down transform capability. FIG. 1a and FIG. 1b illustrate an exemplary representation of such a multilayer unipoled piezoelectric transformer. The device of FIG. 1a and FIG. 1b has certain design and performance limitations. Because the innermost electrode region 101, at the center of the ring or annulus areas 103, are electrically inaccessible all the individual ‘central dot’ layers which comprise region 107 must be uniformly poled in a common direction. This causes the central ring to act as a single piece of ceramic and though it is physically a multilayer construction it is electrically a single poled layer. Each additional layer, or increased thickness, reduces the capacitance of the center dot region thus increasing the impedance of the respective port of the transformer. This increase in output impedance then reduces the effectiveness of the device as a step-down transformer. Additionally, this restriction obviates the ability to selectively control the effective impedance as seen by the input side AC drive voltage which prevents enabling higher power throughput in many situations such as one-to-one or step-down transform ratio applications.
The configuration of FIG. 1a can add additional layers of either alternating or common poling direction interlace. However, irrespective of the number of layers and poling orientation selections, the prior art of FIG. 1a and FIG. 1b is further limited to operate either in Series-in/Parallel-out mode, Parallel-in/Series-Out mode, or Series-in/Series-out mode of electrical configuration. In particular, prior art exemplified by FIG. 1a and FIG. 1b precludes any feasible realization of a Parallel-in/Parallel-out, which creates the ability to create a low output and input impedance, operation, but it is precisely this configuration that is essential to larger voltage step-down applications of piezoelectric transformer constructions that exhibit high efficiencies and high power throughput.
The prior art as exemplified in FIG. 1a and FIG. 1b has another undesirable set of restrictions. The electrode design arrangement is not conducive to either low cost production or scalability in terms of voltage transformation or power level. Increasing or decreasing the number of layers in the design becomes a complicated modeling and redesign challenge. Additionally, no modular approach to layer fabrication or assembly can be applied.
FIG. 2a presents a planar mode single layer piezoelectric transformer in a toroidal form factor. Turning to FIG. 2b, the concept of a Parallel-in/Parallel-out mode operation is achieved utilizing a pair of devices of FIG. 2a, but is done so at the cost of physical separation the piezoelectric elements. Unavoidable variation in resonant frequency, no matter the tolerance of fabrication, will cause such a solution to perform at reduced efficiencies. The variation in resonant frequency between device 201 and device 203, in FIG. 2b, causes the need for a sacrifice to be made when driving the devices from a single source 205. Since the devices 201 and 203 will have two distinct resonant frequencies they can't be driven in parallel efficiently from a single frequency source 205. This problem becomes most pronounced when attempting to parallel multiple elements, as the mean variation in resonant frequency increases, the overall performance of the combined of devices will decrease. The present invention aims to eliminate this issue of variation in frequency by physically coupling the layers as a single mechanical structure. When fully mechanically coupled, the stack itself has a single resonant frequency, thus individual layers in the stack operate at this single resonant frequency, eliminating any reduction in performance from phase cancelation and off-resonant operation.